1. Field of the Invention
The invention relates to a discharge lamp, especially to a discharge lamp of the short arc type which is used, for example, as a light source in UV irradiation treatment in the photochemical industry, in semiconductor manufacture and the like or as a light source in projections, as in a projector or the like.
2. Description of the Prior Art
FIG. 5 is a front view which schematically shows the arrangement of one example of a conventional discharge lamp 40 of the short arc type. The discharge lamp 40 of the short arc type has a bulb 42 and an arc tube 41. An essentially oval discharge space S is formed in the bulb 42. The arc tube 41 has hermetically sealed portions 43 which border the two ends of this bulb 42 and which extend outward from it. In this arc tube 41, a cathode 44 and an anode 45 are arranged in opposed relationship. Furthermore, the arc tube 41 is filled with at least one rare gas in a suitable amount. The arc tube 41 is moreover filled, depending on the application of the lamp, with a suitable amount of mercury together with the rare gas. Here, the rare gases with which the arc tube 41 is filled can be, for example, xenon, krypton, argon and the like. A base 48 is located on the outer end of the respective hermetically sealed portion 43.
The cathode 44 has a tapering part 44A which is shaped essentially like a truncated cone, with a diameter which decreases in the direction to the tip end (anode side), and a body part 44B which borders this tapering part 44A extends to the rear in the axial direction, and for example, is cylindrical. The cathode 44 contains an emitter substance such as, for example, thorium or the like.
In such a discharge lamp of the short arc type 40, when the lamp is started, a high voltage, for example, from a few kV to a few dozen kV, is applied between the cathode 44 and the anode 45, by which an insulation breakdown takes place between the cathode 44 and the anode 45. Afterwards, a transition to an arc discharge takes place and the lamp is operated.
The discharge phenomenon when the lamp is started is described specifically below.
Immediately after the insulation breakdown between the cathode 44 and the anode 45 has taken place, an arc start point is formed on the tip surface 46 of the cathode 44. An arc is formed such that it extends in the axial direction between the cathode 44 and the anode 45. The reason for formation of the arc start point on the tip surface 46 of the cathode 44 is described below.
Because the cathode 44 has essentially the shape of a truncated cone in which the tapering part 44A is present and its diameter decreases in the direction to the tip end, an electrical field is concentrated on the tip area, especially on the edge area on the tip surface 46. In this way, the electrons in the tip area become free more easily. Furthermore, after the insulation breakdown has taken place and the arc has been formed, the cathode 44 reaches its highest temperature at its tip area. As becomes apparent from the Richardson-Dushmann equation, there is a tendency for the thermion emission capacity to increase exponentially according to the temperature increase. The electron emission capacity of the tip area becomes greater than in the other area of the cathode 44. For these and similar reasons, the arc start point is formed on the tip surface 46 of the cathode 44.
However, in a discharge lamp of the short arc type 40 with the above described arrangement, as is shown, for example, in FIG. 6, there is the case in which, when the lamp starts, the state in which the start point P of the arc A is formed on the tip surface 46 of the cathode 44 does not continue in a stable manner, but the start point is moved to a rear position in the axial direction—for example, to the surface position of the tapering part 44A which is away from the tip surface 45, to the surface position of the body part 44B or the like—and that it moves toward the tip side according to the temperature increase of the cathode 44. This means that it happens that the so-called fluctuation phenomenon of the arc A occurs.
When the start point P of the arc A is formed at the above described position, as was described above, the arc A is formed, for example, such that it extends in the manner of an arc along the inner surface of the arc tube 41. In this way, a state is obtained in which the arc A of the inner surface of the arc tube 41 is approached. Or, depending on the conditions of the arrangement and the operating conditions of the lamp, a state is obtained in which the arc A is in contact with the inner surface of the arc tube 41. As a result, the following disadvantages arise.                (1) The contact point of the arc A with the arc tube 41 is subject to devitrification. This reduces the light transmission factor of the arc tube 41. The intensity of the light which has been emitted from the discharge lamp of the short arc type 40 therefore becomes nonuniform. As a result, the illuminance on an article which is being irradiated with light becomes nonuniform. In the case, for example, of an application as a light source in the field of semiconductor exposure, the expected treatment cannot be reliably performed because nonuniform exposure takes place. In the case of use as a light source in the field of projection, an image with sufficient brightness cannot be provided.        (2) By contact or approach of the high temperature arc A, the inside surface of the arc tube 41 is quickly heated. This yields thermal distortion. As a result of this thermal distortion, the discharge lamp of the short arc type 40 is damaged.        
The above described fluctuation phenomenon of the arc A occurs more and more distinctly in the course of repeated use of the lamp (on or off operation). The reasons for this are the following:                (1) During lamp operation, the tip area of the cathode 44 reaches a high temperature of, for example, roughly 2000° C. to 2500° C. The tip area melts, vaporizes and therefore deforms. The degree of concentration of the electrical field decreases.        (2) The emitter substance which is contained in the cathode 44 dries out in the course of repeated use of the lamp. The electron emission capacity of the tip area therefore decreases.        (3) The crystals of the tip area become coarser due to the thermal effect and the grain boundary between the crystals diminishes. In this way, the emitter substance is more poorly guided to the tip area and the electron emission capacity of the tip area decreases.        
Various factors like these and similar reasons overall cause formation of the fluctuation phenomenon of the arc A, since the start point P of the arc A moves more often to a position outside of the tip area 46 of the cathode 44.
In view of this disadvantage, technology has been disclosed (see, for example, Japanese patent disclosure document 2003-257363) in which the following is done:
As is shown, for example, in FIG. 7, for the cathode 50 which has a tapering part 51 and a body 52 which borders this tapering part, in the tapering part 51, a concave part (concave part 55 in FIG. 7), a projection, or the like is formed. This concave part 55 or a projection prevents the start point of the arc from moving in the axial direction to behind the point at which the concave part 55 or the projection is formed. In this way, the formation of the fluctuation phenomenon of the arc and also devitrification or damage to the arc tube are prevented.
However, even when using the technology disclosed in JP-A-2003-257363, there are cases in which the arc start point passes beyond the point at the tapering part 51 of the cathode 50 at which, for example, the concave part 55 is formed, the arc start point moves, for example, to the surface position of the body 52 of the cathode 50, and at this point, the arc start point is formed. Therefore, there is the disadvantage that devitrification or damage to the arc tube as a result of the fluctuation phenomenon of the arc cannot be reliably prevented.